1. Field of the Invention
The present invention relates to an elastic wave filter device and to a communication apparatus equipped with an elastic wave filter device. In particular, the present invention relates to a ladder-type elastic wave filter device and to a communication apparatus equipped with a ladder-type elastic wave filter device.
2. Description of the Related Art
In recent years, with the increasing functionality of mobile communication apparatuses such as cellular phones, it has become increasingly common to mount, for example, GPS (global positioning system) and WLAN (wireless local area network) elastic wave filter devices, in addition to elastic wave filter devices for mobile communication, in communication apparatuses.
In a communication apparatus that includes such a plurality of elastic wave filter devices, intermodulation distortion (IMD) is generated in some of the elastic wave filter devices due to a non-linear effect exhibited by the elastic wave filter devices and as a result the filter characteristics are degraded. For example, sometimes a signal of a certain elastic wave filter device and a signal of another elastic wave filter device are input to yet another elastic wave filter device through antennas. In such a case, in the yet another elastic wave filter device, a combined signal is generated at a frequency that depends on the sum of or the difference between the frequency of the signal of the certain elastic wave filter device and the frequency of the signal of the other elastic wave filter device. In the case where the frequency of the combined signal is within the passband of the yet another elastic wave filter device, the filter characteristics of the yet another elastic wave filter device are degraded. Specifically, in a communication apparatus that includes a WLAN elastic wave filter device, a GPS elastic wave filter device and a mobile communication elastic wave filter device for use in UMTS BAND 5, 6, or 8 or GSM-900 or GSM-850, since the difference between a signal of the WLAN elastic wave filter device (2400 to 2500 MHz) and a signal of the mobile communication elastic wave filter device (800 to 1000 MHz) coincides with a center frequency of signals of the GPS elastic wave filter device (1575, 42 MHz), the filter characteristics of the GPS elastic filter device are degraded due to IMD.
A method of suppressing generation of IMD is, for example, a method in which a series-arm resonator positioned closest to the antenna is divided into a plurality of parts, as described in Japanese Unexamined Patent Application Publication No. 2008-85989. The non-linear effect of a filter has a strong correlation with input electrical power. Accordingly, the higher the electrical power of a load is, the larger the non-linear effect becomes and therefore the more likely generation of IMD becomes. By dividing the series-arm resonator positioned closest to the antenna into a plurality of parts, the area of the series-arm resonator can be increased and the power consumption per unit area can be reduced. As a result, generation of IMD can be suppressed.
In addition, another method for suppressing generation of IMD for example is a method in which a capacitor and an inductor are connected in parallel between a transmission/reception filter and an antenna as described in Japanese Unexamined Patent Application Publication No. 2007-336479.
However, there is a problem in that, when a series-arm resonator is divided into a plurality of parts or a capacitor and an inductor are connected in parallel with each other, the elastic wave filter device is increased in size.